Cathode ray device



oct. 8, 1940. v C J. DAVISSQN 2,217,197

CATHODE RAY DEVICE Filed neo. 5o, 193s v 2 sheets-sheet 1 AAAAA AAAAAA I*YEAH IMA MAA RE C TIF/E R ,IdF/Ing s e e DEFLEC/ON PLATES /A/ VEN TORC. J. 0A V/SSOA/ AT ORN/5y Oct. 8, 1940.

c. J. DAvlssoN CATHODE RAY DEVICE Filed Dec. 30, 1936 2 Sheets-Sheet 2vm IN voLTs c. J. 0,4 l//sso/v Patented Oct. 8, 1940 UNITE CATHODE RAYDEVICE Application December 30, 1936-, Serial No. 118,277

2.4 Claims.

This invention relates to cathode ray devices and more specifically tomethods .of and apparatus for generating, focussing, accelerating andmodulating they intensity of electron beams in cathode ray devices.

An object of this invention is to provide improved cathode raygenerating and controlling means for cathode ray tubes.

Another object of this invention is to provide a novel electron gunsystem for a cathode ray device.

A further object of this invention is to provide an electron lens systemin Awhich apertured plates are used throughout for focussing the beam.

A still further object of this invention is to provide a novelmodulating system for electron beams in cathode ray devices.

A feature of this invention is that within the electron lens system afield free space is defined, in which region the cathode ray beam ismodified.

Another feature .of this invention is that a condensing electron lenssystem is provided for concentrating an intense electron beam upon theplane of a diaphragm having a square aperture therein and a projectingelectron lens system is provided for forming an electron image of thisaperture uponva screen or target.

For illustrating a practical application .of the invention, there ishereinafter described in dedetail a tube which may be brieflycharacterized as a high vacuum cathode ray device electron opticallydesigned for use as a receiver for a 72- line television image. It is tobe understood that the invention in its various aspects is not limitedin its application to any specific type or form of cathode ray devicebut is of quite general application.

Investigation has disclosed that ideally such a cathode ray receiver asthat just mentioned would employ a uniformly bright square spot,modulated in brightness but not in area. It would have a linearbrightness versus modulating voltage relationship over a range at leasta` factor of thirty below maximum brightness, and likewise a lineardeflection versus deflecting voltage relationship over the rangedetermined by the field of the image. Further, there would be nodistortion of the spot due to deflection nor deflection of the spot bymodulation. The size of the image would be n times the size of the spot,'n being the number of lines, so that when the image field isilluminated without modulation, a uniform field would be obtainedwithout evidence of line structure. Moreover, the image would beapproximately as large as the fluorescent screen permits so that thesize of S of the spot is given by the equation:

where D is the effective screen diameter. Finally, provision would bemade for adjusting brightness in operation without changing the linearmodulation characteristics, maximum brightness being as great aspossible. The present invention sufficiently well meets these idealrequirements'.

Briefly stated, the electrode structure of the television receiving tubementioned above is as follows:

A cross-shaped filament serving as a cathode is located between andparallel toa back electrode and an accelerating focussing electrode. Ane g ative potential with respect to that of the cathode is applied tothe back electrode and a positive potential is applied to the firstaccelerating electrode, the effect of which is to produce a uniformfield between the two members to cause the electrons emitted from thefour arms of the cross-shaped filament to traverse 'paths which aresubstantially parallel to the optical axis of the tube. Three diaphragmmembers are located in a metallic cylinder forming a second accelerating`anode. #The potential of this' second accelerating anode is placed at avalue which is positive with respect to that of the first acceleratingvanode. A third accelerating anode comprising lan apertured diaphragm isplaced at a potential which is positive With respect to that of thesecond accelerating anode and is electrically connected `to a conductingcoating on the inside walls of the tube. The distances of the firstaccelerating member and the first and second diaphragms of the secondaccelerating member from the cathode and the relative potentials appliedthereto are calculated so as to cause electrons from the cathode tov befocussed in the plane of the middle diaphragm of the second acceleratingmember.. The final diaphragm of the second` accelerating member (withrespect to the position of the filament) cooperates with the thirdaccelerating electrode to form a projection lens system to focus anelectron image of the aperture in the middle diaphragm upon a screen ortarget. The beam is modulated by a pair of modulating plates which varythe number yof electrons incident upon the aperture in themiddle'diaphragm of the second accelerating anode in accordance with theamplitudes of signals received from a transmitting station. The beamFig. 1 is a schematic diagram of a televisionv receiving tube and itsassociated circuits which embody this invention;

Fig. 2 is a schematic diagram showing the relative spacing of thevarious elements of the electrode system in the tube shown in Fig. '1;

and

Figs. 3 to 6, inclusive, are graphical representations used to explainthe operation .of this in vention.

Referring more specically to the drawings, Fig. 1 shows a cathode raytube and its associated circuits for use as a television receivingdevice. This tube comprises a gas-tight envelope I containing anelectron gun assembly for generating, focussing and accelerating a beam.of electrons, means for modulating this beam, and means for deectingthe beam so that it traverses every elemental area of a field of view ona uorescent screen II located at one end of the tube.

The electron gun arrangement comprises a cathode I2, a back plate orelectrode I3, a first accelerating anode I4, a second accelerating anodeI5, and a third accelerating anode IE which is electrically connected toa conducting coating I7 located on the inside walls of the tube. Broadlyspeaking, the electron gun arrangement comprises two electron focussingor lens systems of the electrostatic type, .one a condensing lens systemfor concentrating a beam of electrons generated by the cathode I2 uponthe aperture in the metallic diaphragm S which is located inthe metallicanode cylinder I5, and a projection lens system for projecting an imageof this electron illuminated aperture upon the screen or target lII.Preferably the shape of the aperture in the diaphragm S is made squareor rectangular.

'I'he cathode I2 is preferably formed in the shape of a cross from asingle sheet of tungsten of the order of .001 inch thick. The oppositeends of this cross are electrically connected together and terminals I8and I9 connected to a suitable source of heating current 20. Aresistance 52 is also connected across terminalsl and I9, the mid-pointof which is connected to ground. By this method of connection theelectrostatic and electromagnetic elds due to filament current andpotential are reduced to substantially zero. For a more completedescription of `a crossshaped element similar to the one describedabove, reference may be made to Patent 2,117,- '709, issued May 17, 1938to C. J. Davisson.

The back electrode I3 comprises a circular plate spaced a short distancefrom the cathode I2 and parallel thereto. The first acceleratingelectrode I4 comprises an apertured circular plate which is located onthe side of the cross-shaped cathode element remote from the backelectrode I3. A positive potential with respect to that of the cathodeis applied to the first accelerating anode I4, while.a negativepotential with respect to that of the cathode is applied to theback-electrode I3. In order to produce a uniform eld between the backelectrode I3 and the rst accelerating anode I4, the spacings of thesemembers I3 and I4 from the cathode I2 are so chosen with respect to therespective potentials applied thereto that the ratio of these potentialsis equal 'to the ratio of their respective distances from plied theretoa positive potential with respect to that of the first acceleratinganode I4. This ,l potential and the spacing and aperture sizes of thediaphragms of the members I5 and I4 are so chosen that the electron beamis brought to a focus in the plane of the apertured diaphragm S. Therelative spacings and potentials applied yto the various electrodemembers will be considered more fully below.

The third accelerating anode I6, which preferably comprises a metalliccircular plate having an aperture therein, is placed at a positivepotential with respect to that of the second accelerating anode I5.members I5 and I6 and the distance between them is so chosen that anelectron image of the aperture in the plate S is formed on thefluorescent screen II, which maybe of any suitable fluorescent materialsuch as, for example, willemite. The third accelerating anode iselectrically connected inside the tube to the conducting coating I'Iwhich serves to make a eld free space betweenthe anode I6 and theiluorescent screen II and at the same time to serve as a return path forthe electrons impinging upon the fluorescent screen II.

In order to cause the electron beam generated by the electron gunapparatus described above to .scan every elemental area of the field ofview on the screen or target II in turn, suitable deflecting means, suchas, for example, two pairs of deecting plates 23 and 24, the axes ofwhich are located at right angles to each other, are provided. To thedeilecting plates 23 are applied dei'lecting voltages ofy framingfrequency cycles) and of saw-tooth wave form to produce the verticaldeection, while deflecting voltages of line scanning frequency (1440cycles) and of saw-tooth wave form are applied to the deecting plates 24to produce horizontal deection of the beam. Any suitable sweep circuits(not shown) may be used to generate these horizontal and verticaldeection voltages. For example, reference may be made to Patent2,178,464, issued October 31, 1939 to M. W. Baldwin, Jr., whichdiscloses a suitable balanced sweep circuit for this purpose.Connections may be made from the balanced sweep circuits to the pairs ofplates 23 and 24 by means of coupling condensers 25 and 26 and 21 and28, respectively, of about 1 microfarad capacity each. Couplingresistances 29 and 39 of the order lof 20 megohms each are respectivelyconnected across the-pairs of plates. The mid-points of the resistances29 and 30 are connected to the third accelerating anode I6 in order thatthe average potentials of the deflecting plates do not deviate from thepotential of the third accelerating anode i6 and thus introducedistortion of the beam with the consequent distortion of the image whichwould be caused by a change in the velocity of the beam. For a fulldescription of the advantages of balanced sweep circuits in cathode raytelevision devices, reference may be made to the above-mentioned Baldwinpatent and also to Patent 2,209,199 is- The potentials applied to thesued July 23, 1940 to Frank Gray, Serial No. 65,606.

The direct current potentials for biasing the vdifferent elements of theelectron gun are pref- 1 erably derived from an alternating currentoscil- 35 of about 1 microfarad each which are connected between vtheseterminals and ground.V

Modulation of the electron beam generated by the apparatus described-above is achieved by applying signals between a pair of modulatingplates M1 and M2 which are located in a eld free space determined bylthe cylinder I and the diaphragms 2I and S, all of these elements beingrplaced at the same potential. The potentials applied to thesemodulating plates are balanced with respect to the potential of thecylinder I5 by means of a resistance 36 of the order of 200,000 ohms,the mid-p oint of which is connected to the cylinder I5, and a balancedmodulating circuit is connected to the terminals of the resistance 36.An input signal is applied (after demodulation by a suitable circuit orcircuits) to the primary winding 31 of a transformer 38, the secondarywinding 39 of which is connected to the input circuit of two tubes 40and 4I which fills ing plates M1.

are connected in push-pull manner to produce a balanced output. Thecathodes 42 and 43 of the tubes 40 and 4I are connected to ground. Anegative bias is applied to the grids 44 and 45 by means of a connection46 from the midpoint of the transformer secondary winding 39 tov a pointon the potentiometer resistance 33 Which is negative with respect to thetap representing ground potential. The output circuits of the tubes 40and 4I include resistances 4l and 48 of about 5,000 ohms each, thecommon terminal of which is connected to the mid-point of the resistance35. The output of the tubes 40 and 4I is applied to the modulatingplates M1 and M2 through'coupling condensers 49 and 50 of about 1microfarad capacity each.

By means of the arrangement described in the preceding paragraph, theaverage of the potentials applied to the modulating plates M1 and M2 isat all times equal to the potential applied to the accelerating anode I5and hence tothat of the diaphragms 2I and S which are connected to themetallic cylinder. Modulationis thus achieved in a field free spaceexcept for the variations introduced by the changes in the potentialsapplied to the modulating plates; that is, if the average oi thepotentials -applied to the plates is always equal to the potential ofthe diaphragms at both ends of 'the plates there is no accelerationimparted to the beam while it is being modulated.

Modulation is achieved by deflecting the electron beam more or less uponthe square hole in the apertured diaphragm S. Electrons are deectedtowards the more positive of the modulat- By thus controlling the numberof electrons incident upon the square hole in the apertured diaphragm S,modulation of the brightness of the beam can be achieved. The crosssection of the beam at the square aperture in the plate S has, ofcourse, an unequal distributio-n, as explained eleswhere herein, thedistribution being a maximum atthe center and theoretically falling offto zero at infinity. Strictly speaking, therefore, the beam is neverfully deiiected off the aperture and the distribution within theaperture is never exactly uniform. This unequal distribution, however,is rarely, if ever, discernible to the observer viewing the imagescreen.

Specific details of the tube generally described above will now be givenin order toI fully describe a tube which has been used in practice. Asquare hole in the plate S was made equal to .006 inch by punching. Sofar as a square spot on the fluorescent screen in concerned, its sourceis this .006 inch hole and what happens to the electrons before theyreach the hole has no effect upon the size and shape of the imageproduced on the fluorescent screen by the projector lenses in thediaphragms 22 and I6. The optical analogue is the projection lantern.The inside wall of the Pyrex glass cylinder is coated with aquadag andthis coating I1 is shown connected to member I6 by a spring 5I which isreleased by burning out a lament during pum-ping. During the lsealing-inprocess this spring should not be permitted to scrape against theaquadag.

As an aid to visualizing the electrode assembly, it should be mentionedthat between the cylinder I5 and the element I6 and between pairs ofelements I3 and I4 and I4 and I5 are placed cylindrical glass insulators(not shown) of the `salme diameter, which diameter is somewhat greaterthan that of the element I5 and less than that of the other elements.The electrode structure is supported on three metal rods passing throughperipheral apertures in elements I3, I4, and I6 and a flange or flanges(not shown) on element I5. These rods are bolted to a metal band whichis clamped to the base of the tube. to Fig. 2 it will be seen that theelectrically independent parts are the back electrode I3 (designated inFig. 2 as P1), the filament F, the first accelerating anode P1, thesecond -accelerating anode comprising .the apertured diaphragms P2, SandP2 (which are all placed at the same potenti-al) M1 and M2, the thirdaccelerating anode P3 (which is electrically connected to the aquadagconducting layer I'I) and each ofthe four deilecting plates. Thus atotal of twelve leads is required. All metal parts are aluminum.,

except P1 which is of nickel and the filament F which is of tungsten.Metal ilanges protect the cylindrical insulators which prevent or renderharmless the accumulation of charges.

The tube is thoroughly pumped. practical to degas the metal parts byhigh frequency, but the tube is baked for several hours at 460 C.;par-ts adjacent the filament are heavily bombarded; and the tube isiinally sealed off from the pumps at a temperature of about 100-C. and apressure of 7 107 millimeters of mercury. p

The proper `values of the potentials applied to the various electrodemembers an-d the distances of these members from each other will now beconsidered.

The principal physical consideration made use of in arriving at thedesign of the television receiver tube described `above is that in anaxially symmetrical electric lfield the convergence of a paraxial beamof electrons satises the diierential equation Referring It is im- Beforedefining the symbols in this equation it will be well to state that beamin the foregoing sentence has a special restricted meaning and toexplain what this meaning is.

Let it be imagined that electrons are streaming along the axis of anaxially symmetrical fieldnot necessarily all on the axis but near it. Ifthey have all come from the same, emitting surface their speeds as theycross any given plane normal to the axis will be sensibly the same.'I'heir trajectories will not, in general, be straight lines. If we'draw tangents to their trajectories at the points at which they cut agiven plane these will not, in general, meet in a point. If they do meetin a point the stream of electrons constitutes a beam in the sense inwhich the word is here used. The point of meeting may be in front of theplane (in the direction of motion of the electrons or back of it; it maybe on the axis or oi .it.` The only requirement is that the tangentsmeet in a point on or near the axis. If this condition is satisfied thestream of electrons is a cal considerations that if the tangents drawnat one cross-section meet in a point then the tangente drawn at allother cross-sections also i tive or negative.

meet in a point. The convergence of the beam at any given cross-sectionis kdefined as the recprocal of the distance from the cross-section tothe point at which the tangente meet, and this is the quantity which isrepresented by the above equation symbol c.r

The convergence of a beam may be either posi- If at a givencross-section the tangents are parallel, the convergence of the beam Vatthis cross-section is zero. lf the trajectories are intersecting oneanother at a given cross-section, the distance from the plane to thepoint of intersection of the tangents is zero and the convergence of thebeam is infinite.

No actual stream of electrons is strictly speaking a single beam. If thestream originates at a themionic cathode the electrons leave the cathodewith small initial speeds variously directed. The

tangents to their trajectories at the emitting surface do not meet in apoint and it follows from theoretical considerations already alluded tothat at no cross-section will they meet in a point.

If, however, not all of the electrons in the stream are considered butonly those which leave a given very small element of the cathode, it isclear that-these constitute a beam at their point of origin, since herethe tangents intersect. This being true they constitute a beam at allcrosssections further along their paths. The vwhole stream of electronswhich travels along the axis of the held is an assemblage of suchelementary beams originating at the cathode, or may so be regarded. v

There are, however, other Ways of regarding the stream. The electronswhich leave a ilat cathode in some particular directionr may be thoughtof as constituting a beam. They form a beam of definite convergence atthe cathodenamely, zero-and so have a definite convergence elsewherealong their path. The whole stream may be regarded as an assemblage ofbeams of this kind if desired.

What the differential equation does is to show how the convergence or" abeam-any beamvaries along the axis of an axially symmetrical electricl'ield.

Let c stand for the convergence of the beam at thecross-section`throughthe point a onthe axis, e be the distance along the axis measuredon the axis relative to the potential of the cathode which is ordinarilyassigned the value zero. Then V'=dV/dz, (the first derivative of V withrespect to e) and V"=d2V/dz2, (the second derivative of V withrespect toa). What the differential equation says is that at every point along theaxis the rst derivative of the convergence of Va beam (dc/dz) is equalto c2-cV/2V|V/4V, where the quantities in this expression are those forthe particular point or particular cross-section (particular z) underconsideration. The equation is derived from the fundamental laws ofelectrodynamics. It applies to every beam no matter how constituted andwhether on the axis or somewhat off.

lf the value of V at al1 points along the axis is' known, that is, if Vis known as a function of z, (V=f(z) this function can be written intothe general equation and thus a particular differential equation interms of c and e can be obtained. The solution of this equation will beof the form F(c,2) :a constant. It will show the convergence of any beamat any value of a in this particular field, provided only itsconvergence at some one value of e' is known. A beam may, of course,have any convergence at any value of e, but once its 4convergence at onevalue of z is fixed, its convergence at every other value of z is iixed.Mathematically this amounts to xing the value of the constant in therequation F(c,z) =a con stant. If it is known that at .ai theconvergence of the beam is c1, then the constant is F(C1,e1). F(c.z)'=F(ci,21) then describes a particular beam in the particular field.

Thus in the cases considered, the beams starting from elements of thecathode are beams in4 which the convergence is OO at 2:05 thus c1=-,21:0. The beams made up of .electrons leaving along parallel lines arebeams of Zero convergence at 2:0.

Axially symmetrical fields are produced by ap` plying potentials toelectrodes which are coaxial gures of revolution-such as platescontaining circular holes, cylinders, etc., strung along a common axis.When the configuration of the electrodes is fixed and the potentialsapplied to them are. known, the potentials at points along the axisV=f(z) can (in principle) be calculated.

ReplacingV in the general differential equation by a) the appropriatediiferentia1 equation in c and e is Iobtained. Solving this andevaluating the constant of integration, a description of the beam isobtained. Y

Let there now be considered any set of parallel plates' containing nottoo large circular holes. It can'be shown that when a beam passesthrough the field about a hole in an arrangement of this kind, itsconvergence vis increased by an amount Ac=(V2'-T/T1)/4V, Where V is thepotential of the plate and V2 and V1 are the values of :iV/dz on theemergence and incidence sides of the plate respectively-more strictly V2and V1 are the values CIV/dz would have on the two sides of the plate ifthe diameter of the holes were reduced to zero.

When a beam of light passes through a lens of focal length f itsconvergence is increased by an amount l/f. It is natural therefore toregard the field about a hole in a plate as a lens for electrons,L offocal-length f=4V/(V2'-V1')..

Thus if adjacent plates in the arrangement being considered are at 2:22and 2:21, and if their potentials are V2 and V1, V between the plateswill have` ythe value V2-V1)/ (azi-e1). If a beam-entering the regionthrough a hole in plate P1 has a convergence c1, then, by the law juststated, it will have at the second plate convergence c2, such that l2V21/2 1 2V11/2 Lcm1/2+ vf Wt v' These two laws, the lens law and thelaw just stated are all that are required to calculate the convergenceof a beam at any pointalong its path through any number of plates at anyassigned potentials-provided, of course, its convergence at some onevalue of z is known.

Thus let it be supposed that the cathode is a flat surface normal to the'axis of the field. All electrons which leave this surfacenormally (withzero lateral components of initial velocities) might be considered as abeam. These constitute a beam of convergence zero at 2:0. In the eld tothe rst plate the convergence will change in such a way as to keepconstant When` the beam passesY through a hole in the first plate, itsconvergence is increased by an amount Ac: (V2'-V1) /4V1 as alreadyexplained. The convergence of the beam is increased vfrom :0 to c=Ac. Inthe second region the convergence again satisfies the law 2V1/2 Vl TheValue of the constant is obtained by using the known value of theconvergence of the beam as it enters the region. Applying the law, theconvergence of the beam at the second plate is now calculated andthenits convergence after passing through the second plate and so onthrough as many plates as is desired.

l The beam being considered is that composed of electrons which leavethe flat cathode normally (Without initial lateral velocity). The beamcomposed of all the electrons leaving a small surface element might havebeen considered instead. In this case the convergence of the beam at 2:0is instead of Zero. The `calculations are carried through as in thepre-vious case, but the convergences found for the beam are, of course,everywhere different.

- It is important to understand how these two kindsof beams are related,the beam of normally emitted electrons on the one hand, and the beamsemitted fromusmall surface elements w+ constant on1the. other. Itis'clear that a beam of the latter type shares some electrons With thenormally emitted beam; all of the electrons emitted normally from agiven small surface element belongy .naturally to both-beams. Not onlyis this true, but it is also true that of all of the electrons emittedfrom the small element, those emitted normally are in a Way the mostimportant. The reason isthat the initial energies of the emittedelectrons are small compared to the energy they acquire in the fieldonly a short distance from the cathode. Because the initial energiesvare comparatively small, the paths or trajectories traversed by theelectrons from a small element are none of them so very different fromwhat they would be if the initial velocities were Zero.. The actualtrajectories V,all lie close to the trajectories of an electron whichleaves the elementfrom rest. This isparticularlytrue of the trajectoriesof electrons which leave the element with normalinitial velocity, but nolateral velocity, for

these start off from the surface in the same direction as that taken byan electron initially at rest;

It is because of these considerations thatthe electrons emitted from asurface element, although they leave the element in all possibledirections, are, at a short distance from the cathode,formed into anarrowbeam. At the center of this elementary beam are the electronswhich left the surface normally. The elementary beam is not, however,sharply bounded. The situation is rather this, that the electronsemitted normally from the element follow a certain trajectory throughthe field, beingnear the axis of the field in some places, further fromit in others, and perhaps at certain points crossing it. Thetrajectories of the other electrons emitted fromv the same elementcluster about this principal trajectory. YThe density of these othertra.-

.jectories lis greatest at the center of the' cluster, declines andapproaches zero asymptotically with distance from the center.A It is aprobability distribution, like bullet marks about the bulls-eye of atarget and for this reason not sharply bounded.

Itis important to realize that these trajectories which cluster aboutthe principal trajectory are thetrajectories of the beam of electronswhichrleave the small element. Where the convergence of this beam goesto infinity, the cluster contracts to a point, `all trajectories of the-cluste-rpass through a point and this point is, of course, on theprincipal or central trajectory. g

- The trajectories of normally emitted electrons are all principaltrajectories, each `is the principal trajectory of all trajectoriesstarting from some surface element. Thus, one Way of describing thewhole stream ofelectrons is to give the convergence of the beam ofnormally emitted electrons for every value of z and in addition togiverthe convergence of the elementary beams also for everyvalue of a.All elementary beams have the same convergence at a given Value of ebutnever the same convergence as the beam of normally emitted electrons,the beam of electrons pursuing principal trajectories. It is impossible,for example, to make both convergences infinite at the same value of e.It is impossible, for this reason, to bring al1 of the electrons from anextended cathode to a point focus. The principal trajectories can bemade to pass through a point, but where this happens the convergence ofthe elementary beams is not infinite. The convergence of the elementarybeams can be made infinite, but

Where this happens the principal trajectories are spread out, and do notcome to a focus;

` This latter condition is that in which a real image of theemittingsurface is formed. The elementary beams .are brought to a focus(convergence made infinite) on a fluorescent. screen; the

electrons which started from asmall element of the cathode are broughttogether on a small element of the screen, but Where this happens thebeam of principal trajectories is necessarily not ltube described aboveat the square aperture in the diaphragm S. The arrangement of ribbonfilament, backing plate and apertured lens plates is designed to producean intense focal spot in the plane of the square aperture.

'Ihe primary consideration inthe design of this part of the tube is toarrange for a crossing of the principal trajectories in this plane.Further back, it was explained how the convergence of a beam at anypointV or cross-section along the axis of a eld determined by any set ofarbitrarily disposed plates containing circular holes can be calculated,the plates being at any potentials. While this is possible, it is'notactually done. A person really starts with the idea of bringing theprincipal trajectories to focus somewhere out along the axis, and seeksas general expressions as possible for the positions and potentialsoflens plates which will produce this result. It is found that thesuitable conditions are not at all unique. It is found that there mustbe at least two lens plates if the focus is to be produced in a eld freespace. But it has been discovered that the plates may be anywhere at allbetween the filament and the point of focussing. No matter where theyare placed the principal trajectories will cross at the chosen pointwhen appropriate potentials are applied to the plates, or rather whenthe potentials applied to the plates have an appropriate ratio.

For best' results, certain vother factors need to be taken intoconsideration. One of these is that V13/2/z12) must not have too low avalue or trouble will be encountered with space charge (V1 is thepotential of the rst plate and e1 Vits distance from the filament).Another is the maximum current density in the focal spot. The choice ofgeometry must be such as to have this as high as possible. Still anotheris the matter of the solid angle occupied by the electrons which passthrough the square aperture. point in getting a great lot of electronsthrough the square hole if they are going to be spread over a. solidangle much greaterv than that subtended by the'lens which is designed toform them into an image on the fluorescent screen. There is also thequestion of the convergence of the elementary beams. The arrangementshould be such, if possible, that the elementary beamsenter the eld freeregion with a positive convergence. This is needed to make the focalspot small and also to avoid enlarging the solid angle occupied by theelectrons beyond the square hole. There is also the requirement thatsuflicient room be left between the second lens plate and the squarehole There is no.

to accommodate the'modulator plates. The eld between these movesthefocal spot oi and on the square hole. Besides the geometry of thecondenser system there is also the question of its scale.

Considering now the beam from the plane of the square hole to thescreen, this hole is effectively a new source just as in light optics.An entirely new set of beams is set up. The elementary beam from thispoint onward is made up of the electrons which stream through a givensmall element of the aperture. At the aperture it is a certain distance01T the axis of the system and has a convergence of There is now noprincipal trajectory. The electrons in these new elementary beams aredistributed more or less uniformly in a solid angle only slightly largerthan that subtended by the projector lens, toward which they aredirected.

The design of the projector lens is based on the same considerations aswere previously used in the design of the condenser lens system. Quite alarge lens aperture is used in order to gather in all of the electrons`which come through the square hole, but if the aperture is too large apoor image' of the square hole is produced on the screen because of lensaberration of one sort or another.

The projector lens is really a combination of two lenses, one ratherweak lens (P3) and one strong positive lens (Pz). The apertures areadjusted to make the F-number of each lens as great at least as '7 or 8.This is about the F-number (ratio of focal length to diameter) of thepositive lens. The F-number of the negative lens is greater. Fromprevious experience it is known that a lens of the type here used ofF-number '7 or 8 produced sharp images.

Regarding the magnification of the projector system what is wanted onthe screen is a spot 30 mils square. This means that the square apertureof which the spot is an image must be 30/M mils square, where M is themagnification of the projector lens system.

The really important quantitatively expressed relationships which havebeen exactly applied in the-making of tubes in accordance with thisinvention are:

(I) 'Ihat the eld about a circular hole in a plate is, for'electrons, alens of focal length (II) That in a uniform eld the convergence of abeamof electrons traveling parallel (or antiparallel) to the directionof the field, or nearly so, varies in such a way that the expressionremains constant.

These laws are deduced from the general differential equation l where Zrefers to the distance from the filament.

'I'hen when V2:Vi Z2:Zi, the hole in plate P1 constitutes a positiveelectron lens and that in plate P2 a negative electron lens. Aspreviously discussed, it is possible to calculate the electrondistribution on the plane S for any voltage ratio and geometry. Moreusefully, it is possible to calculate the geometry required to give amaximum intensity-at-peak of the electron distribution on S, subject tothe restriction that all of the electrons incident on the square holeshall be limited to a certain angular spread about the axis, such thatthey will also pass through the circular hole in plate Pz', whichconstitutes the rst lens of the projector system. There is the furtherrestriction that the magnification of the projector lens system shall beabout 5 which, in conjunction with the overall length of the tube helpsto determine the distance between S and Pz; and also a restriction isimposed on the F-numbers of the projector lenses. Finally, there are therestrictions of space charge limitation of emission from the filament,and, perhaps, of power dissipation by plate Pi which, however, is not alimit in this tube. The geometry of Figs. 1 and 2 is an optimum design,the only more or less arbitrary restriction being that on the F-numbersof the electron lenses. The recommended spacing be- .tween the elementsof the electron gun, as shown in Fig..2, is as follows; between the backelectrodes P1 and the filament F, 2 millimeters; between the filament Fand the first accelerating anode P1, 6 millimeters; between the firstaccelerating anode Pi and the diaphragm P2 ofthe second acceleratinganode, 4 millimeters; between the diaphragm P2 and the diaphragm S, 10millimeters; between the diaphragm S and the diaphragm P2', 25millimeters; between the 4diaphragm P2' and the apertured diaphragm P3,comprising the third accelerating anode, 15 millimeters; and between thethird accelerating anode Ps and the fluorescent screen II,`30centimeters. With these spacings and with voltage ratios which will begiven below, a very ne intense spot is produced on the fluorescentscreen Il.

The remaining design problems are those of the modulating plates and ofthe deflecting plates. The former are made as large as geometry willpermit and are adequately spaced; their sensitiv'- ity will be discussedbelow. The latter nd their sensitivity fixed by the distance to and thediameter of the iiuorescent screen. t

'Ihe diameters of the circular plates have not been mentioned for thereason that these are not critical. v

In operation, the potentials V1, V1', V2 and V3 are supplied from thealternating current `rectifier 3l which is connected to the Voltagedivider circuit 33 of about 1 megohm total resistance. Changes in outputvoltage do not change voltage ratios, except temporarily due tocondensers in the circuit. Electron optical solutions are always interms of voltage ratios; changestin absolute value over wide ranges ofvoltage do not affect the' electron focussing. n

The focussing ratio l@ V2 for the projector lens system is 4 2.- Thiswas determined by visual observation of the spot at low intensity, thatis, with filament current low. This ratio is not dependent upon anyother factor, either number of electrons in the beam, absolute value ofvoltage, or other voltages, such as V1, Vi', or modulating and deectingvoltages, within the limits of operation of these factors.

The correct Voltage ratio V 1' ZL" Vl Z1 is similarly practicallyindependent of all other factors. By geometry l Z1 3 approximately, buta slight filament displacement can make relatively large changes in theratio, and it appeared thatthe correct ratio was about 1/4. At aboutthis ratio the current i1 to plate P1 is but half of the total filamentemission (temperature-limited), a condition usually satislied. The shapeof the ii versus Vi curve conrms'the value 1A, and visual observation ofshort images of the filament, ywhich may be formed on the screen ifdesired, also leads to the value of 1A. But in any event, for the spot,what happens between the filament and S is of no importance. beamcurrent should be obtained and the secondary consideration is that i1should be as small as practicable to reduce the power dissipated on theplate P1. Fig. 3 shows beam current as a' function of for (1)temperature limited and (2) `space charge limited emission. It will beseen from these curves that a value of :Yi v Vi of approximately 0.25 or0.26 is the optimum value.

It seems advisable to specify the method employed in measuring the beamcurrent. Ideally, a Faraday box which could be moved to intercept thebeam would be used. It is observed, however, that plate Pa and theaquadag coated tube itself comprise a good Faraday box, and the currentmeasured in the lead to P3 is the beam current provided (A) that allelectrons passing through S and lens P2' also pass through lens P3, (B)that the current measured is not effectively reduced by current to thedeecting plates and (C) the leakage currents are taken into account. Allof these conditions can be fulfilled so that the beam current i3 istherefore the current to plate P3 and the aquadag coated bulb.

The next set of curves; as shown in Fig. 4 shows beam current as afunction of the condensing voltage ratio E t V1 when V1 is heldconstant. Curve I is taken for avalue of V1=v152 Yvolts vand a value ofIs` (the The criterion is that maximum CII filament current) :4.40amperes. Under these conditions the current to the plate P1 is limitedby space charge. Curve 2 is taken for a Value of V1=24O Volts and avalue of Ia=3.60. Under these conditions the current to plate P1 islimited by the temperature of the lament. 'Ihe maximum of these curvesis attained at a value of V1 for constant V2 might be considerablydifferent but Fig. 5 shows the optimum to be nearly the same. In takingthe curve shown in Fig. 5, V2 was held constant at 1,000 volts, lamentcurrent being 4.40 amperes and the ratio of -Vi' to V1 being heldconstant at 0.25.

Thus far, the data have shown that there exist optimum values of thevoltage ratios -Vl' V2 17T-0.26, Tf1-7.1 and that these ratios areessentially independent of filament emission and of absolute values ofvoltage. Thus, the condensing lens system is focussed so that maximumintensity occurs at S. Also when V3 V2 is equal to 4.2, the projectorlens system focusses this spot on the fluorescent screen.

The data on deflection and on modulation will now be considered. The twopairs of deecting plates are distinguished by subscripts h and o; thelatter are nearest to the screen and produce vertical deflectionwhilethe former produce horizontal deflection, the tube being mountedhorizontally. If all plates, both deflecting and modulating, areinsulated, it will be necessary to specify the average potential of eachpair and the potential difference between them. The average potentialsare designated The potential differences are designated Vh, Vv, Vm, thesubscript m, of course, applying to the modulating plates. Thus deectingand modulating voltages refer to potential differences, and not topotential as measured from the zero potential, namely, the filament. Thespot is distorted when either Vn or Vv is different from Va;consequently, in all tests Vm is made equal to V2. In some eases V2 maybe made greater than Vm as for example in an arrangement shown in Patent2,168,760, issued August 8, 1939 to C. J. Calbick.

The deflection sensitivities Sh and Sv may be dened by the equationswhere dn and dv are the deflections in centimeters` on the fluorescentscreen produced by deecting voltages Vh, Vv, respectively. lIdeallythese sensitivities are constant over the deflec- More genwill definethe sensitivities over the screen` range. Conceivably Sv may also dependon dh, since the electrons pass between the horizontal deflecting platesbefore passing between the Vertical deflecting plates. Since Vb: Vu: V3:

the question of what happens vwhen these conditions are not satisfieddoes not arise. The sensitivities Were vdetermined by photographing thespot in several deflected positions. From this photograph Sh=68.9,Sv=64.0, and 6=015. Here 0 is the angular departure from mutualorthogonality. The horizontal sensitivity is constant to within theaccuracy of measurement, about` one-half of 1 per cent; the verticalsensitivity changes from 63 on one edge of the screen to 65 at theother, i. e., it shows a measurable variation across the screen. Overthe television image field, its extreme variation is less than 2 percent and is quite undetectable in the image field. By making Vv analternating potential, the spot sweeps out a vertical line, which isstraight for all values of Vn; conse quently Sv does not depend on Vh.

Fg. 6 shows a modulation curve taken with V2=1000 volts and the filamentcurrent Ia=`4.0 amperes this curve being plotted with beam current inmicroamperes as ordinates versus Vm in volts as abscissae. It will beobserved that the peak of the modulation curve does not occur at Vm=0but at a slightly positive value and also that a longer linear region isavailable on the right than on the left side of the maximum. This regionis therefore selected for operation.

The above described tube made in accordance with this invention producesa Well defined high intensity square `spot on the screen when the beamis stationary. Television images produced by this tube are relativelyVfree from distortions commonly present. The tube is especially adaptedfor monitoring or in other places Where especially high quality resultsare desired.

Various modifica-tions may obviously be made without departing from thespirit of the invention, the scope Vof this invention being defined inthe appended claims.

What is claimed is:

1. In a cathode ray device, means for generating a beam of electrons, acylindrical electrode located in such a position that its axis coincides with the longitudinal axis of said device Which also passesthrough the center of said beam generating means, said cylindricalelectrode having three apertured diaphragms located transversely of theaxis thereof and each being placed at the potential of the cylinder, theaperture in each diaphragm being placed to surround the axis of saidcylinder, and a pair of plates between said first and second apertureddiaphragms to modulate said beam by varying the portion thereof Whichpasses through the aperture of the second of said apertured diaphragms,the plates being located on opposite sides of said beam.

2. In a cathode ray device, means for generating a stream of electrons,an apertured diaphragm, a screen, Vmeans for concentrating said streamupon the aperture of said apertured diaphragm, means for deflecting saidstream `in accordance .withavariable modulating voltage to v causevarying-portions of said stream to pass throughsaid aperture inaccordance with the variations of said modulating voltage, and meansforiorming `an electron image of said aperture uponwsaid screen wherebythe shape of said image varies in accordance With said'modulatingvoltage. 1

3. In a cathode ray tube, a cathode for emitting electrons, ananodevcomprising a metallic cylinder having threel diaphragms, eachdiaphragm arranged in a plane which is transverse ofthe axis of thecylinder and each vhaving an aperture located so asA to surround saidaxis, means for vimparting a positive potential to said cylinder'vwithrespect to that of said cathode, means including the apertured diaphragmnearest said cathode for focusing electrons upon the plane of the middlediaphragm, a screen, and means. including the remaining apertureddiaphraginfor focusing an image of said middle diaphragm on said screen.

4. In a cathode ray device, a pair of electrodes, a; cathode placed,between said pair of electrodes, and means for applying potentials tosaid electrode members and to said cathode of such value that saidcathode is in a substantially uniform field between said electrodemembers.

5. -I n a cathode ray device, an electron emitting` electrode, a secondelectrode, an apertured plate on' theside of said electron emittingelectrode remote from said second electrode, means for applying-,anegative potential with respect to the' potential o f said electronemitting electrode to said lsecondelectrode, and means for applying apositive potential with respect to that of said electron." emittingelectrode to said apertured plate-the ratio of the magnitudes of thepotentials-applied to ythe second electrode and to the aperturedV platebeing made substantially equal to the ratio of the distances of thesemembers from theelectron emitting electrode.

(i. Anf electron gun arrangement comprising a back electrode, a cathode,a first accelerating ano de, a secondgaccelerating anode, and a thirdaccelerating anode. 1

'7. An electron gunsystem comprising a planar cathode, a back electrodeparallel to said cathode, a. rst acceleratingl anode comprising anapertured diaphragm, a second accelerating anode comprising Va vmetalliccylinder having three apertured' ,diaphragma each diaphragm locatedtransversely of the agis of said gun system and spaced-'along it, meansincluding said first accelerating. anode and the diaphragm of saidsecond accelerating anode nearest said irst accelerating anode for'concentrating the beam of electrons :upon .the plane of the middlediaphragm ofsaid' second accelerating anode, a third accelerating ranodecomprising a metallic lapertured diaphragm; and means including saidthird accelerating anode and the apertured diaphragm in g saidy secondaccelerating anode farthest removed from said first accelerating anodefor projecting an electron image of the aperture in said middlediaphragm of said second accelerating anode upon said screen.

8. In a cathode ray device, a cathode, a back electrode, an acceleratingelectrode, means for applying apositive potential to said acceleratingelectrode with respect to the potential of said cathode, and means forimparting a negative potential with respect to the potential of saidcath g ode to said back electrode, the ratio of the voltage applied tothe back electrode to .that applied to the accelerating electrode beingapproximately 1 to 4.

9. In a cathode ray device, a ribbon iilament, an anode, and a platelocated on the side of said filament remote from said anode, the planeof said plate being parallel to the plane of said lament.

10. In a cathode ray device, a tungsten ribbon filament, ankanode, and aplate located on the side of said filament remote from said anode, theplane of said plate being parallel to the plane of said filament.

11. In combination, an apertured member located in a space free fromrfields caused by potentials applied to elements Voutside said space, anelectron emitting element, a back electrode, and two apertured elementsbetween the apertured member and the electron emitting element, vone ofthe latter defining aboundary of said space.

l2. In a cathode ray device, a ribbon lament, an anode, and a platelocated on the side of said filament .remote fro'msaid anode, the planeof said plate beingy parallel to the plane of said lament, and means forapplying potentials to the lament, the plate vand the anode, saidpotentials being of such value thata substantially uniform eld existsbetween the plate and the anode.

13. In a cathode ray device, a pair of electrodes, a cathode inthe formof perpendicularly crossed elements placed between said pairofelectrodes, and means ,for placing said electrode members and saidcathode atsuch potentials that said cathode is in a substantiallyuniform field between said electrode members.

14. An electron gun arrangement comprising a back electrode, a cathode,va iirst accelerating anode, a second accelerating anode comprising ametallic cylinder-havingjthree apertured diaphragms therein eachdiaphragm having an aperture surrounding the axis of said cylinder, anda third accelerating anode.

15. In an electron optical system comprising means includinga pair ofelectrodes on opposite sides of a` flat cathode Vfor forming a beam ofparallel electron rays, a rst apertured diaphragm, means Vforfforming aeld free space around said firstY apertured diaphragm, a screen ortarget, electrostatic means. for converging said beam so that ,in theabsence of deflecting fields substantially all` of said electrons passthrough the aperture insaid diaphragm, means for deflecting saidy beamwithin'said field free space in accordance with signals so as to varythe number of electrons passing through said aperture, and meansincluding a pair of apertured diaphragms for causing said electrons toform a diverging beam to form an enlarged electron image of the aperturein said first apertured diaphragm upon said screen or target.

16. In an electron optical system comprising means including a pair ofelectrodes on opposite sides of a fiatcathode for forming a beam ofparallel electron rays, an apertured diaphragm, a screen, means forconverging said beam so that in the absence of` deflecting fieldssubstantially all of said electrons pass through the aperture in saiddiaphragm, means for moving said converging beamin accordance withsignals so as to Vary .the numberofelectrons passing through saidaperture, and 'means' for causing said electrons to form a divergingbeam to form an enlarged electron image of said aperture upon saidscreen.

,17. In an electron optical system comprising means including a pair of.electrode members on opposite sides of a at cathode for forming a beamof parallel electron rays, an apertured diaphragm, means for forming afield free space around said apertured diaphragm, a screen or target,electrostatic means for converging said beam so that in the absence ofdelecting fields substantially all of said electrons pass throughl theaperture in said diaphragm, means for defleeting said beam within saidfield freespace in accordance with signals so as to vary the number ofelectrons passing through said' aperture, and means for causing saidelectrons to form a diverging beam to form an enlarged electron image ofsaid aperture upon said screen.

18. In an electron optical system comprising means including a pair of'electrode members on opposite sides of a fiat cathode for forming abeam of parallel electron rays, an apertured diaphragm, means forforming a eld free space around said apertured diaphragm, a screen ortarget, electrostatic means for converging said beam so that in theabsence of deflecting elds substantially all `ef saidV electrons passthrough the aperture in said diaphragm, means for deflecting said beamwithin said eld' free space in accordance with signals so as to vary thenumber of electrons passing through said apertures, and electrostaticmeans for causing said electrons to form a diverging beam toform anenlarged electron image of said aperture upon said screen.

19. An electrode arrangement of an electron be-am device comprising acathode member, an

I anode member and a backing member for said cathode, said members beingspaced apart with said cathode between the others of said members, andmeans for applying potentials to said members respectively having therelationship that the ratio of the potential applied to said backingmember to that applied to said anode is substantially equal to the ratioof the distance between said cathode and said backing member to thedistance between said cathode and said anode.

20. An electrode arrangement of an electron beam device comprising acathode member, an anode member and a backing member for said cathode,said members being spaced apart with said cathode between the others ofsaid members, and means for applying potentials tol said membersrespectively having the relationship that the ratio of the potentialapplied to said backing member to that applied to said anode issubstantially equal to the ratio of the distance between said cathodeand said backing member to the distance between said cathode and saidanode, said backing member and said anode being substantially parallelto each other throughout their extents.

21. A cathode ray device comprising means for generating a beam ofcathode rays, an apertured element through the aperture of which saidbeam passes, a pair of deilecting plates, one on each side of the beamemerging from said aperture and equidistant therefrom, means forapplying modulating potential across said plates to deflect said beam asmall amount in accordance with variations in said modulating potential,a second apertured element adjacent said plates with its aperturepositioned to receive said beam as it emerges from the space betweensaid plates and to pass a portion only of said beam for certain Valuesof said modulating potential so that the number of electrons passing isunder control' of" the modulatingpotential, and means for causing theaverage of the potentials applied to said able modulatingvoltage tocause varying por-V tions ofrsaid stream to pass through said aperturein accordance with the variations ofv said modulating voltage, saidstream having a substantially rectangular cross-sectional areaimmediately after passing through said aperture with one dimension only'of said area varying in accordance with said modulating voltage, andmeans for forming an electron image of said aperture upon said screen,whereby the shapeof said image varies in accordance withsaidf modulatingvoltage. f

23. In a cathode ray device, means for generating a stream of electrons,an apertured diaphragm, the aperture in said diaphragm having at leastone straight boundary across which said stream is swept by deflectingmeans, a screen, means for concentrating said stream upon 'the apertureof said apertured diaphragm, means for deilecting said stream inaccordance with a variable modulating voltage to cause varying portionsof said stream tofpass through said aperture in accordance with thevariations of said modulating voltage, said stream having asubstantially rectangular cross-sectional area immediately after passingthrough said aperture with one dimension only of said area varying inaccordance withy said modulating voltage, means for forming an electron"image of said aperture upon said screen whereby the vshape of said imagevaries in accordance with said modulating voltage, and additionaldeflecting means for deflecting said stream after it is modulated tocause it to scan said screen inparallel elemental strips extending inthe direction of said varying dimension of the cross-sectional area ofsaid stream.

24. A cathode ray device comprising means for generating a beam ofcathode rays, a first apertured element through the apertureof whichsaid beam passes, a pair of deecting plates, one on each side of thebeam emerging from said aperture and equidistant therefrom, means forapplyingrmodulating potentialacross said plates to deflect said beam asmall amount in accordance with Variations in said modulating potential,a second apertured element adjacent said plates with its aperturepositioned to receive said beam as it emerges from the space betweensaid plates and to pass a portion only of said beam for certain valuesof said modulating potential so that the number of electrons passing isunder control of the modulating potentiahmeans for placing the rstapertured element at the same potential as the second apertured element,and means for causing the average of the potentials applied to saidplates to be at all times substantially equal to the potential of saidapertured elements.

CLINTON J. DAvIssoN.

